Frequency stabilizing system



July 5, 1949. w.E. BRADLEY ET AL 2,475,074

FREQUENCY STABILIZING SYSTEM FiledAug. 31. 1'944 7/ IN V EN TOR.

Patented `Iuly 5, 1949 2,475,074 FREQUENCY s'iABILrZ'I'NG SYSTEM Tompkins; Springfield Tdwnsliilgj Delaware County?, Pa., assignors, by'mesneassignn'i'ents,

poration of Pennsylvania 2' Claims. (C1. 25o-soi' The!` presenti invention relates to: automatic fiequency;` stabilizers and lmorel particularly to a; frequencyrstabilization arrange-nient for ave-v locity *modulated*V electron `bearnrultrahigh fre-- quency .'osci-llator.

In? 'f radar beaconf-systems the :receivers are arranged toi receive* continuous iwaves, ahdaccord'- ingly aclocar/-oscillatoris provided forr the receiver.` Infor-der to provideiproperreceptionit is necessary tov maintain substantiallyconstant the' frequency` of-'os'cillat-ons produced Lby, the local` oscillator. Affveloci'tyr-modulated' electron beam ultra f' highi frequency'fvacuumtube suitable for suoli*operation'r 'has the'f characteristic that' the frequencysof oscillationsfdependsprimarilyupon the :dimensions "'of the i resonant cavity' used asl tlie 'oscillator tankl circuit 'and l`the potential sup-x plied-toithe repel'ler: f Accordinglylit isproposed toi-provide fsonie arrangement whereby "the 'potential supplied'iinay be'vari'edor adjustedfauto`A matinal-ly i to ybrina-r they `frequency" of oscillationl back tofthe'desiredfvalue.- Such'an arrangement of course'f-would compensatef'ffor any variation inthepotential@suppiied and also anyfvariations inthe'ifrequency-fof oscillation causedioyv a-'chang'e inthe constants foffthe resonant xcavi'tykused as'i theli `oscillator` "tank"?circuit.'`

Inflaccordanceflwith ther-present invention'j the frequency ofoscillatien suppliedby "the oscillator isl compared periodically -with a lfrequency stand-'ii interval in orderto fpro'videthe fcornparisonfwithv amrfthenris supplie-dad' aibaianceallpoianty "de:

creaseinlthepower transmittedfly the'frduen'cyf standard@resonanti1V cavity; I Th'fbal'a'nced poll' larity' detector i' is" varranged' to "ontrolf 'means for ivaryingg: in appropriate'fmagnitudeand direc-f the ultra high frequency@oscillatorf-so-thatt its' frequencyds l adjusted' vin theH proper' direction?.A

"cavity-,' the fre-'7 It,ther"e'for'e`, is iair'obuiect ofthe" present in'- venton'4 to lprovide an improved method of and' apparatus for maintaining relatively 4 stable the frequency" of arf ultra" high frequency' oscillator utilizing;- a` velocity modulated electron beam tube.

It is' a'further' object of the' present invention tot 'provide'fafrequencystabilization circuit 'which automatically at periodic' intervals operates: to compare` the" frequency of 'the' oscillations gener'- ated" with 'a'v frequency standard.'

Itisanother objectof the pes'ent'invention to'provide an arrangeinentfor periodically frequency modulating the" ultra' vhigh freclu'e'ncy os` cillator energy'an'd for'comb'aringi the modulated frequencyV with' the" uninodulated oscillations in order thatthe mean frequency' of the' oscilla-v tion `can Joeadjustedperiodicallytv I Other andfurther'objects ofthepresent invention*'subsequentlyi will become' apparent by reference" to` the following description "taken in connection f with' the` accompanyingy drawing wherein Figure'l showsa circuit arrangement embody-v ingthe `present invention; and

Figure 2 is` afgraphical representation explana: toryfortheoperation of the"circutshow`n in Figure'l.-

Inaccordalnce with the present invention there isprovided" a local oscillator fori va receiver 'of the type 'heretofore mentioned. Thislocal oscillator utilizes ave'lo'city modulation' electron bearn'tulo'eA having-ia'resonantI cavity' which is' coupled to an' output:transmission` means Yandy which? also 1 supplies"energytoadreduency standard. One

manner in which such an arrangement may be constructedis shown'in Figure 1. 4

IniFigre 1 there is shownavelocity modulated electron beam vacuu'm'tube I I ofthe type having' a'heater I2; a heated'cathode I3, a control elec* trodeldya repeller I5, and'grids I 6 which are to'fbe'fconnected *to "ai "resonant cavity I T. The resonant cavity l T andthe control electrode I4 are supplied with'potential from a'conductor I8 which: isconnectedto' thepostive terminal of a sourcebf direct currentpote'ntial." The cathode I3--sy connected to adirect` current potential of lower valuethanthat supplied by the conductor I8=. The repeller I5 is" at a lesser positive"po Jtentialrobtainedthrough a 'series resistor 2|v fron'ra voltage divider circuit'extending between grounc'i'l andv the" positive terminal vof a 'direct cur-i rent'source. 'I'hevoltage'divider circuit includesv resistors 22, 23, vacuum'tube 24;'and a'cath'od'e resistorr2i." The `resist`or`23' 'has an adjustable 3 contact connected to the resistor 2|. A small by-pass capacitor |9 is connected between ground and the repeller |5, the purpose of which subsequently will become apparent.

A portion of the energy generated within the tank circuit |1 of the Vacuum tube is coupled by suitable means such as a coaxial line 29 to a resonant cavity 3| which is adjusted or tuned to the frequency at which it is desired to have the oscillator operate. The coaxial line 29 may be provided with a branch 32 so that ultra high frequency energy may be supplied by the tank circuit resonant cavity |1 to the receiver with which the oscillator is associated. The resonant cavity is coupled to a microwave detector 33, which may be a crystal detector. When the oscillations generated by the vacuum tube and the tank circuit |1 are of a frequency corresponding to the desired frequency to which the resonant cavity 3| is adjusted, the resonant cavity 3| will transmit the maximum amount of energy to its output detector 33. This is illustrated in Figure 2 by the curve 34 which shows that at the frequency Fo which represents the desired frequency of operation, the maximum energy is transmitted through the resonant cavity 3|. Whenever the oscillator frequency deviates from Fo, as for example F1, which is less than the desired frequency, the resonant cavity transmits a smaller amount of energy. Similarly when the frequency is higher than the desired frequency Fo, as for example F2, the power transmitted through the resonant cavity 3| is also less than maximum.

In order to determine as to Whether the power transmitted by the frequency standard to the microwave detector has changed, the frequency of the oscillator is periodically shifted a small amount by a variation of the potential appearing at the repeller |5. In the embodiment shown in the drawing this variation of the repeller electrode potential is produced by a high audio frequency pulse generator. Generally the oscillator will be operating at its normal frequency, which is the frequency of the frequency standard resonant cavity 3|. Each time that a pulse is supplied by the high audio frequency pulse generator to modify the potential at the repeller electrode of the velocity modulation electron vacuum tube the frequency of the oscillator Will be shifted momentarily to a lower frequency. This shift in frequency may, for example, comprise deviation existing for approximately ten per cent of the time while for the remaining ninety per cent of the time the oscillator is operating at its normal frequency.

Since the potential change at the repeller electrode |5 of the vacuum tube is in the form of a pulse, the resultant deviation in frequency will change the power transmitted by the frequency standard 3| to the microwave detector 33 which causes the output of the microwave detector 33 to produce a pulse, the polarity of which depends upon the mean frequency of the oscillator with respect to the frequency of the cavity 3|. This is illustrated in Figure 2 from which it will be seen that if the frequency of oscillation prior to the action of the pulse generator is at a frequency F1 the pulse Will cause a momentary frequency deviation which will produce a momentary drop of the detector output. On the other hand if the average frequency of the oscillator is F2 the action of the pulse produces a momentary rise of detector output.

In order to periodically shift the frequency of the ultra high frequency oscillator a high audio frequency pulse generator, which may be of the asymmetrical multivibrator type, is provided utilizing a pair of vacuum tubes 35 and 36. The anodes of the vacuum tubes 35 and 36 are supplied from a suitable source of potential through series or coupling resistors 31 and 38 respectively. The grid of the vacuum tube 35, which is provided with a grid-to-cathode resistor 39, is coupled by a capacitor 4| to the anode of vacuum tube 35. The grid of Vacuum tube 36 which is provided with a grid-to-cathode resistor 42 is coupled by a capacitor 43 to the anode of the vacuum tube 35. The anode of the vacuum tube 35 is connected to a series resistor 44 which is connected to the juncture of the resistor 2| and the repeller |5 of the vacuum tube Since the high audio frequency pulse generator which includes the vacuum tubes 35 and 36 has its output connected to a point on the voltage divider which supplies potential to the repellerelectrode l5 of the vacuum tube each periodic pulse supplied by the generator operates to produce a momentary shift in the frequency generated by the vacuum tube and the tank circuit |1. The capacitor I9 controls the magnitude of the potential change at the repeller |5 due to the pulse supplied by the pulse generator.

These pulses produced at the output of the microwave detector 33 in response to the pulse action on the oscillator may be negative or positive pulses dependent upon the mean frequency, and these pulses are supplied to a pulse ampliiier. After the pulses have been amplified they are supplied to a polarity detector subsequently to be described. The microwave detector 33 is connected to the grid circuit of a vacuum tube 45 which may include a series resistor 46 and a grounded grid resistor 41. The cathode of the vacuum tube 45 is connected to ground and the anode is -connected through a suitable coupling resistor 48 to a source of anode potential. The output of the vacuum tube 45 is coupled by a capacitor 49 to the grid of another vacuum tube 5|. The vacuum tubes 45 and 5| together with their associated circuits comprise a two stage pulse amplifier, the output of which is fed to a balanced polarity detector circuit. The vacuum tube 5| has a grounded grid resistor 52, and a grounded cathode resistor 53 which is by-passed by a capacitor 54. The anode of the vacuum tube 5| is connected through a coupling resistor 55 to the source of anode potential. The anode of the vacuum tube 5| is coupled to the balanced polarity detector circuit by similar capacitors 56 and 51.

The polarity detector, which preferably is in the form of a balanced polarity detector comprises a pair of diode detectors 58 and 59 arranged so that the cathode of one diode detector 58 and the anode of the other diode detector 59 are connected to ground. The remaining electrodes of the diode detectors 58 and 59 are connected to the coupling capacitors 56 and 51. A pair of resistors 6| and 62 interconnect these electrodes, and the common juncture therebetween is connected to the grid of the vacuum tube 24 through grid damping resistor 26, and is by-passed to ground by capacitor 63. The balanced polarity detector circuit is arranged to control the potential supplied to the grid of the vacuum tube 24 thereby to vary the impedance of the vacuum tube 24 in accordance with variations in power transmitted by the resonant cavity 3| as detected periodically.

The balanced polarity detector has a property such as' the t the-output across the diode 59. The two resistors Ili and-162 y"comprise a voltage'divider so that the common ljuncture "therebetween has a potential below ground equal to about half of the potential appearing at the anodev of the diode 58.

If the frequency deviation produced by a pulse corresponds to the pulse 64lshown in'Figure l2.

the 'microwave detector '33 supplies a negative pulse such as 65 to the pulse amplifier. The'pulseV thus amplified appearing at the anode of the vacuum tube l develops no potential across the vdiode k58 but develops a potential across the ldiode `59.- The potential appearing at the cathode of diode 59 is a positive direct current potential Vwhich is substantially equal to the amplified peak 'a pulse voltage; and one-half of the direct current voltage appears at the juncture of the resistors 6I and 62. The voltages thus appearing at the juncture of the resistors 6| and 32 are supplied n tothe control grid of the Vacuum tube 24 which is connected 1n the Voltage divider circuit supplying voltage to the repeller electrode l5 of the vacuum tube Il. The balanced polarity detector "therefore develops a positive :direct current potential in response to negative Apulses received from theanode of the Vacuum tube 5l, and negative direct current potential in response to positive pulses received at the anode of the vacuum tube 5l thereby to modify the action of the control tube 24 whenever the oscillator is too high or too low in frequency.

From the above explanation of the function and operation of the balanced polarity detector,

it will become apparent to those skilled in the.

art that such an arrangement may find other application in other types of circuits. Such circuit arrangements might include a polarity detector of this typein various measuring apparatus and in control systems.

When the oscillator comprising the vacuum tube il and the tank circuit I1 is operating at a frequency corresponding to the desired frequency Fo as determined by the resonant cavity 3l, the slight deviation of the frequency due to the action of the pulse supplied by the asymmetrical multivibrator circuit produces no significant change in oscillator output as detected by the microwave detector 33 as is shown in Figure 2. If it is assumed that the frequency of the oscillator is less than the desired frequency, as for example at F1, the frequency shift produced by the pulse is as has been indicated at 64 in Figure 2. Thus for a short interval the frequency of oscillation has been reduced so that the power transmitted by the frequency standard resonant cavity 3| is less than heretofore with the result that the microwave detector 33 supplies to the pulse amplifier vacuum' tubes 45 and 5l what corresponds to an amplitude modulation indicated by the negative pulse G5 in Figure 2. The frequency modulation produced by the pulse supplied by the asymmetrical multivibrator shifts the frequency of oscillation by an amount not exceeding one megacycle. This variation which corresponds to an amplitude modulation in a negative sense is amplified and supplied to the balanced polarity detector diode rectiers 58 and 59 to produce a more positive voltage as applied to the grid of the vacuum tube 24 so as to make more negative the potential supplied to the-repeller I5 vof the vacuum tube H, 2t'vhichin-turn increases the mean oscillator frequency.

It'may now be assumed that at a subsequent period arpulse supplied by the asymmetrical multivibrator'produces a frequency modulation when the oscillator is at a higher frequency F2 as indi- -c'ated at 66 inliigiue 2. Since this frequency change 6 6 shifts the vfrequency of oscillation supplied to the'resonant cavity 3l in the direction towardFo there is an increase in the power suppliedv to the microwave detector 33 which corre- 'sp'ondsv toa positive amplitude modulation inditial of'therepeller l5 causes the oscillator frequency to be'reducedto the frequency Fo. The

vacuum tube 24 therefore operates as a variable resistor in lthe voltage divider circuit which includes the resistors '22,"23and 25.

" Whereas the embodiment of the present invention' heretoforeshown and described has indicated a periodic'pulse generator as the means for producing periodic frequency changes or frequen- ''fcy modulation of the oscillator energy, it will be appreciated `other wave forms may be utilized. The frequency standard resonant cavity coupled to theltank'circu'it ofthe resonant cavity of the ultra high frequency oscillator in each case will r indicate a change in the power transmitted by the frequency standard cavity.

If the periodic change in `oscillator frequency,

',forcomparison purposes, is brought about bya sinusoidal wave, certain modifications as are apparent to those skilled in the art may be made in the'circuitbet'ween the microwave detector and the kvoltage divider which supplies potential to the'repeller of the vacuum tube oscillator. With asinusoi'dalvariatior'i` ofthe oscillator frequency,

the microwave detector will produce a sinusoidal wave which may be compared with a sinusoidal wave supplied to modify the frequency of the oscillator by any one of a number of devices as, for example, a phase detector. As a result of this comparison a bias may be generated to modify the action of the vacuum tube in the voltage divider in a manner similar to that heretofore described.

In order to further facilitate an understanding of the embodiment herein disclosed, reference will be made to certain specific elements utilized in a device of this sort. The vacuum tube Il may be of the Velocity modulated electron beam type which is exemplified by such vacuum tubes as designated by the numbers 707,726 and 2K28. With such vacuum tubes a potential difference of approximately three hundred volts is applied between the cathode I3 and the cavity Il. The repeller is operated at a potential between fty and two hundred volts negative with respect to the cathode. In one embodiment the pulse amplifying tubes 45 and 5l are combined in a GSL'ZGT vacuum tube and the diode rectifiers 58 and 59 are combined in a 6H6 diode detector. A GSN'YGT or 6J5G is used as a control tube 24. The asymmetrical vibrator pulse generator circuit utilized a SSL'ZGT. In that circuit the resistor 39 for example had a value of one megohm,

the resistor 42 had a value of twenty-two thouausm sand ohms. Capacitor 4l had a value of .00015 microfarad and 43 had a value of .000015 microfarad and the resistors 31 and 38 each had a value of fteen thousand ohms and the anode supply potential was in the vicinity of two hundred volts. The resistors in the voltage divider circuit 22, 23, and 25 had va1ues of ten thousand, twenty thousand, and six hundred eighty ohms respectively. The resistors 2| and 44 were twenty-five thousand, and four hundred seventy thousand ohms respectively. The by-pass capacitor I9 had a value of .00015 microfarad. A potential is developed by the diode rectier appearing between the junction of resistors 6| and 62, each having a value of one hundred thousand ohms, and ground. The series resistor connected between the resistors 6I and 62, and the grid of the vacuum tube 24 had a value of two hundred twenty ohms and the by-pass capacitor 63 had a value of .1 microfarad. The values of other components shown in Figure 1 were such as are customarily selected by usual engineering practice and will not be enumerated for the sake of brevity.

While for the purpose of illustrating and describing the present invention, it has been mentioned that the oscillator shown is suitable for use in a local oscillator in a radar beacon system, it of course will be appreciated that other applications of this oscillator might be made and that the invention is not to be limited to the particular use mentioned nor to the specific circuit arrangements or values given as examples with a view of facilitating the understanding of the invention. The invention is set forth with particularity in the appended claims and accordingly is to be understood that such variations and modication in the circuit arrangements and in the instrumentalities employed may be made without departing from the spirit and scope of the invention defined in the claims.

This invention is hereby claimed as follows:

l. In a system for stabilizing the frequency of an ultra high frequency oscillator, a cavity resonator coupled to the output of said oscillator, said cavity resonator being continuously resonant at the desired operating frequency of said oscillator, means for periodically and impulsively shifting the frequency of said oscillator slightly in one direction, means for detecting any resultant change in the output of said cavity resonator, and for detecting the direction or sense of such change, and means for adjusting the operating frequency of said oscillator in accordance with the detected change and its sense.

2. In a system for stabilizing the frequency of an ultra high frequency oscillator, a cavity resonator coupled to the output of said oscillator. said cavity resonator being continuously resonant at the desired operating frequency of said oscillator, means for periodically shifting the frequency of said oscillator during short intervals which constitute a small percentage of the total operating time, means for detecting any resultant change in the output of said cavity resonator, and for detecting the direction or sense of such change, and means for adjusting the operating frequency of said oscillator in accordance with the detected change and its sense.

W'ILLIAM E. BRADLEY. HOWARD E. TO-MIPKINS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,198,226 Peterson Apr. 23, 1940 2,222,759 Burnside Nov. 26, 1940i 2,245,627 Varian June 17, 1941 2,280,824 Hansen et al Apr. 28, 1942 2,337,214 Tuniek Dec. 21, 1943 2,358,545 Wendt Sept. 19, 1944 Re. 22,587 Varian et al. Jan. 2, 1945 2,374,810 Fremlin May 1, 1945i 2,375,223 Hansen et al May 8, 1945 2,404,568 Dow July 23, 1946 

